yi PREFACE. 



the study of nervous degenerations has proved an additional source of 

 information in regard to their deep-seated connections. Although the 

 anatomical data obtained in this way must be insufficient by themselves 

 to determine the functions of a part, yet they are of material aid in 

 the contrivance and execution of physiological experiments, and often 

 indispensable for the explanation of their results. Furthermore, the 

 study of the vaso-motor nerves and nerve centres has reached a devel- 

 opment which makes it almost a special department of nervous physi- 

 ology, and requires a more extended treatment than heretofore. 



The method of examination by microscopic sections has also been 

 found of advantage in Embryology. It show r s the form and position 

 of the organs at their earliest period of development, and enables the 

 observer to trace their subsequent changes with greater precision than 

 formerly. The most primitive embryonic structures are still those which 

 present the greatest difficulty in their study and interpretation ; but 

 increased facilities of research are constantly adding to our knowledge 

 in this respect, and reducing the number of doubtful or disputed points. 

 In the present work, as a general rule, topics which are uncertain or 

 incomplete have been treated with comparative brevity, a greater space 

 being devoted to those which are demonstrated by satisfactory evidence. 

 The number of wood-cuts has been somewhat reduced, and many have 

 been replaced by new ones, intended either for the illustration of recent 

 discoveries or as improvements on those of the former edition. 



NEW YORK, December, 1881. 



